The present invention relates to a fuel assembly, and more particularly to a fuel assembly suitable for enhancing fuel utilization, which is to be installed in a boiling water reactor (BWR).
Under the condition that a fuel assembly to be used in the BWR is installed in a reactor core, non-saturated cooling water or coolant is flowed between fuel rods through holes of a lower tie plate of the fuel assembly. As the coolant is flowed between the fuel rods from a lower portion thereof to an upper portion thereof, the coolant is heated and boiled to form a two-phase flow to be discharged from holes of an upper tie plate. Therefore, a light water that is neutron moderator is generally distributed so that the amount thereof is decreased from the lower portion to the upper portion within the fuel assembly. Also, it should be noted that the actual distribution of the coolant in the axial direction depends upon the mutual effect with the power distribution. Namely, an average void fraction of the reactor core determined by such distribution controls the neutron moderation effect of the overall reactor core and the power distribution.
Recently, in order to realize the effective use of uranium resource and the power cost reduction thereof, it has been proposed to enhance an average enrichment of the fuel assembly to provide a fuel assembly with a high burnup. The high burnup of the fuel assembly causes a burnup reactivity to increase, which is needed for continuing the operation among the reactor during one operational cycle. In the BWR, it has been proposed to control the reactivity mainly by adjusting the amount of burnable poison, i.e., gadolinia and the reactor core average void fraction. This adjustment of the reactor core average void fraction is performed by operating the reactor with a high void fraction during the operational cycle from the initial stage to the nearly end thereof, at which the neutron moderating effect is small, whereby the controlled amount of reactivity is increased by an absorption of neutron into uranium 238. The adjustment is performed, inversely, by using a low void fraction at the operational cycle end. A method for adjusting the reactor core void fraction is called "spectral shift operation". In that method, since plutonium 239 converted from uranium 238 is effectively used as a fuel in the final stage of the operational cycle, such effective use of plutonium 239 as well as the reactivity control effect to enhance the fuel economical property. The spectral shift operation method is divided into a method for changing the flowrate of the coolant during the operational cycle and a method for changing the axial power distribution during the operational cycle.
The latter method is disclosed in U.S. Pat. No. 4,587,090. According to this method, a difference in reactivity between the upper region and the lower region of the fuel assembly due to the difference of the enrichment and a difference in reactivity between the upper region and lower region due to the difference of the burnable poison amount are effectively utilized so that the power distribution of the fuel assembly during first half of the reactor operation cycle is deformed downwardly and the power distribution of the fuel assembly during second half of the reactor operation cycle is deformed upwardly.